Physical and chemical characterisation of crude meat and bone meal combustion residue: “waste or raw material?”
Introduction
As a result of the recent bovine spongiform encephalopathy (BSE) crisis in the European beef industry, the use of animal by-products is now severely restricted. Indeed, the transmissible spongiform encephalopathy agent (SPE agent), also named scrappy prion, is responsible for fatal neurodegenerative diseases in animals and humans [1], [2], [3], [4]. Nowadays, brain, spinal cord, tonsils, etc. and sick animal corpses are considered as high risk wastes and must be incinerated. Safe animal wastes (meat and bones) coming from slaughterhouse waste are mixed, crushed and cooked together. After the cooking process, tallow is extracted and the remaining residue, known as meat and bone meal (MBM) or animal flour, is sterilised before being safely disposed off [5], [6], [7]. This sterilised product is called low risk MBM. Since November 2000, low risk MBM can no longer be used to feed cattle but can be incorporated in food for pig, poultry, fish or animal pets. Their import to European Community (EU) and their export from EU is banned. Nowadays, the over production of low risk MBM must be eliminated or safely recycled [5], [6], [7], [8], [9], [10].
Among the different processes studied, valorisation of MBM can be realised by a thermal degradation treatment (incineration or pyrolisis) [8], [9], [10], as they are readily flammable fuel (approximately 17,000 kJ kg−1) [9]. During high temperature combustion (over 800 °C), thermal energy is recovered and proteins such as prions are destroyed (as all organic matter is converted to CO2, H2O, etc.) [1], [2], [3]. Co-incineration in cement kilns is the most common way, for MBM destruction, used in France. They are mixed with cement compounds (calcium, silica, alumina, etc.) and heated over 1500 °C to produce the clinker. In England, the use of MBM dedicated incinerator is developed. Incineration plants are set up in Glanford, Wyminton and Widnes with a joint capacity of 205,000 tons of MBM/year (ton = Mg).
In France, for example, 850,000 tons of MBM are produced each year but actually only 45% can be burned by cement plants [6]. The remaining 55% are stored waiting for further destruction or valorisation. Ashes produced by meat and bone meal combustion represent up to 30% of the original weight. Thermal treatment of the entire MBM production would lead to an important amount of ashes (nearly 250,000 tons/year). Therefore, the fate of meat and bone meal combustion residue stocks is of major environmental concern. Large storage costs for wastes classified as dangerous are also an economic burden.
As MBM combustion residues mainly arise from bone combustion, they contain a high amount of phosphate and calcium, two major constituents of bone. In our search for new valorisation ways we focus on phosphoric acid production, phosphate source for industry, agricultural soil enrichment, heavy metals immobilisation in soil or water, etc. as developed for other phosphate rich materials (rocks, hydroxyapatites, bone char, etc.) [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. We report here a study on the physical and chemical characterisation of meat and bone meal combustion residues. The thermal behaviour of crude MBM was followed by thermogravimetric analysis (TGA). The impact of combustion temperature on the structure of ashes was followed by powder X-ray diffraction. The resulting ashes were characterised by elemental analysis, powder X-ray diffraction (XRD), particle size distribution, specific surface area (BET), scanning electron microscopy (SEM) and energy disperse X-ray analysis (EDX).
Section snippets
Materials and methods
Low risk sterilised meat and bone meal were provided by Fersobio. MBM were burned twice by calcination in an electric furnace programmed to reach 550 °C at 2°/min. During combustion, MBM particles melted and sticked together. The first combustion gave a black residue (carbon rich). This residue was mixed manually before a second combustion in order to complete decomposition and obtain clear ashes.
Thermogravimetric analysis was performed with a Setaram TG-DTA92 analyser, in a platinum crucible,
MBM combustion
In our experiments we used low fat MBM coming from slaughterhouse waste. These products are dehydrated (110 °C/4–5 h) and sterilised (133 °C/20 min/3 bar) according to European standards by Fersobio. Resulting low risk MBM still contains water (3–8 wt.%) and large amount of fats (10–14 wt.%) and other organic compounds (25–35 wt.%) according to low risk MBM producers [6].
Thermal analysis of MBM is realised under air (combustion) and argon (pyrolysis) atmosphere with 3 °C/min temperature increase rate (
Conclusion
The results show that meat and bone meal combustion residues are calcium (30.7%) and phosphate (56.3%) rich compounds, mainly a mixture of Ca10(PO4)6(OH)2 and β-Ca3(PO4)2. Significant levels of sodium (2.7%), potassium (2.5%) and magnesium (0.8%) are also observed. Ash particles are relatively small, from a few millimetre to micrometre, with almost 90% smaller than 1 mm.
Thermal conditions of MBM combustion (between 550 and 1000 °C) induce a wide range of structural modifications (crystallisation
Acknowledgements
The authors wish to thank Fersobio SA for providing meat and bone meal used in this work. We would also like to thank the Chemistry Department of “IUT Paul Sabatier” for financial support and for offering accesses to some analytical equipment, L. Orts and T. Scandella for laboratory assistance.
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